1. Field of the Invention
The present invention relates to a drive device for a lens barrel and a diaphragm shutter which incorporates a stepping motor in a photographic lens assembly used in a photographic camera.
2. Description of the Prior Art
Various proposals have heretofore been made with respect to a drive device in which the motion of a diaphragm or a diaphragm shutter in a photographic lens assembly for a photographic camera is controlled by means of a stepping motor as a drive source (hereinafter referred to simply as "ST motor").
For example, U.S. Pat. No. 4,472,039 discloses a drive device for a diaphragm, while U.S. Pat. No. 3,882,522 discloses a drive device for a diaphragm shutter.
The former patent relating to a diaphragm driving device discloses the technical concept that stator means and rotor means constituting an ST motor are respectively formed in a hollow shape, such hollow rotor means being coupled with a diaphragm driving mechanism, and the hollow rotor being rotated forward and backward, thereby driving a diaphragm.
The latter patent relating to a diaphragm-shutter driving device discloses the technical concept that cylindrical ST motor means is disposed in the vicinity of a diaphragm shutter, such diaphragm shutter being driven via gear means by the motion of this ST motor means.
In the actual operation of film exposure, the above-mentioned diaphragm driving operation must be completed within a short time after the shutter has been released. However, if the rotor means is formed in a hollow shape as described previously, that is, if the hollow portion is used as the optical path of a photographic lens assembly, the rotor means is unavoidably increased in size. In consequence, it is hard for the rotor means to quickly start due to its increased size, while it is hard for it to quickly stop due to an inertia force acting on the rotor means. This causes the problem that the diaphragm driving operation cannot be completed within a short time.
On the other hand, if the cylindrical ST motor means is used, it is possible to solve the above-described problem relating to the starting or stopping response of the rotor. However, for example, when the lens barrel and the diaphragm shutter are both to be driven by the motion of the ST motor, the provision of such motor means requires a large space and the overall size of the photographic lens assembly is unnecessarily increased.